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REMOTE CONTROL BY BOB YOUNG Multi-channel radio control transmitter; Pt 2 This month, after a long hiatus, we continue the de­scription of the Mk.22 transmitter by presenting details of the encoder module. This is a great deal more flexible than was envisaged in the June 1995 issue and is still based on discrete ICs rather than a microprocessor. Here it is at last! What can I offer in my defence for the long delay? Nothing much except that the size and complexity of this project has grown alarmingly and has necessitated calling in additional help in order to get it finished. I am indebted to Dean Herbert (Microherb Electronics) for the design of the basic encoder module presented here. Once again, I set out to design one construction of the basic 8-channel transmitter. This transmitter is intended as a companion to the Silver­tone Mk.22 AM receiver published in the December 1994 & February, March & April 1995 issues of SILICON CHIP. However, it will also act as a replacement for almost any AM PPM transmitter currently on the market. While most transmitters on the As can be imagined, this transmitter did not fall out of a tree but came as a result of months of gruelling proto­ typing and missed deadlines. thing and instead ended up with something completely different; the best transmitter that Silvertone has ever produced. The complete transmitter consists of four PC boards: RF module (AM), basic 8-channel encoder, an expansion PC board with an additional 16 channels, and a configu­ration module. In the next few articles, we will be dealing with the design and 54  Silicon Chip market have a moulded plas­tic case, the new Silvertone Mk.22 has a more rugged powder-coated aluminium case measuring 170 x 140 x 47mm which is quite compact. Apart from being more robust, the metal case is a desir­able feature as it reduces the possibility of interference (third-order intermodulat­ion) from other transmitters close by. Packed into the case is a glittering array of features. Most are routine, common to all modern R/C systems but some are completely unique. As noted previously, it is expandable from 8 to 24 channels and has capabilities for channel alloca­tion, servo reversing, gain control, dual rate, servo travel length adjustment, pseudo endpoint adjustment and mixing on every channel. Also included are four programmable on-board mixers (two inverting and two non-inverting), two programmable on-board toggle switch control modules, fully programmable front panel switches and controls, and an expansion port for a configuration module of which there are four at the moment: F3B, Helicopter, Aerobatics and the very exciting and completely unique formation flying module. These modules plug into the configuration port (mix expand) and configure the transmitter into a task-oriented unit. Finally, as a topping for this culinary delight, we add a dash of exclusive Silvertone relish: frequency interlock and mixed mode dual control (buddy box). As can be imagined, this transmitter did not fall out of a tree but came as a result of months of gruelling proto­ typing and missed deadlines. By far the most difficult task was keeping the system user friendly. A great deal of work has been done on the development of the input networks to each channel and much care devoted to maintaining a completely identical board layout so that mastering one channel results in the mastery of all chan­nels. For example, there is only one type of front panel switch – a SPDT toggle fitted with a 3-pin socket. Thus, any switch may become a retract switch, dual rate switch, mix in-out switch etc, depending on which set of header pins it is connected to. The mix select switch only calls for a 2-pin socket, so any pair on a 3-pin socket will suffice. The sockets on the switches provide an added advantage in that the action of the switch (UP-ON) may be quickly and easily reversed (DOWN-ON). In other words, the front panel switches are fully program­ mable. Alternatively, the switches may be replaced by shorting links on the header pins for permanently installed features. Thus, for example, coupled Aileron/Rudder (CAR) may be installed perma­nently with a shorting link or set up to operate in the switch in/out mode from the front panel. On the other hand, Flap/Eleva­ tor auto compensation is more usually installed as a permanent feature and thus a simple shorting link on the header pins will suffice. Likewise, any channel may be programmed either to be pro­ portional or switched and there are two toggle switch modules onboard for this purpose. The major compromise in the system came about as a result of reducing the number of potentiometers to be adjusted and the number of shorting link combinations available. This was achieved by making the gain control pot programmable and is probably the most clever feature in the user interface. Thus, we ended up with only a single potentiometer to adjust for each channel, which in turn may be the servo travel volume adjust (ATV), endpoint ad­justment, dual rate set pot, mix ratio set pot or whatever, depending upon how the channel is programmed. As a result of this simplification, certain combinations tend to compromise the action of this pot. A good example is the simplification that took place in the dual rate programming. Originally the NORMAL/DUAL RATE programming pins TB1, TB3 etc consisted of six pins arranged in two rows of three; a very cumbersome programming arrangement with many combinations. Now the way we achieve 120% servo travel is to remove the 33kΩ input resistor from circuit, in the GAIN VARY setting. Thus, the NORMAL range is 1-2ms and with the 33kΩ resistor removed, 0.9-1.1ms. By placing the 33kΩ input resistor on the control pot side of TB1, it was possible to reduce the programming to a simple 3-pin plug which allowed the use of an SPST switch for remote operation. The trade-off is that the dual rate pot will actually increase instead of decreasing the servo travel as it approaches full clockwise rotation. This comes about because VR1 effectively shorts out R2 as it approaches the clockwise termi­nal. Thus, when setting the dual rate throw, starting from full clockwise rotation will give 120% servo travel. As the pot is rotated anticlockwise, this will drop back to 100% then on down to 20%. All of this will be explained in detail in future issue. Whilst this action is a little unusual, it is still dual rate even if it does go higher than normal throw. It is only tra- encoder circuit. Circuit operation The basic encoder follows the design philosophy pioneered in the early 1970s which culminated in the Signet­ ics NE5044. It uses a multiplexed ramp generator IC3b to generate standard pulse position modulation (PPM) – see Fig.1. Neutral for all 24 channels is set by a single pot, VR2, associated with IC1b. This feature represents a significant cost saving in transmitters with more than four channels. IC4 is the 8-channel multiplexer, a 4051. In a full system, there are three of these which will allow 24 channels, via the expansion PC board. IC4 samples each control input sequentially until all inputs are examined and then there is a pause (sync pause) before the process begins again. The rate at which Packed into the case is a glittering array of features. Most are routine, common to all modern R/C systems but some are completely unique. dition that states that dual rate must go down from the normal control throw. Another good example of this sort of compromise is the situation that arises when programming for dual rate combined with coupled channel mixing; CAR, for example. In this case, the Aileron gain set pot becomes primarily the dual rate set pot and the mix ratio adjustment is set on the auxiliary mix pot which is part of the four on-board mixers. This feature was a particularly difficult one to achieve, for in the beginning I could not switch the mix ratio with the dual rate switch. By utilising the spare pins on the MIX EXPAND port, I found the programming combination I required. This called for the pins on the MIX EXPAND port to be double-sided and we will cover this point in detail in the fol­lowing articles. Now we have full mixing with dual rate on the mix ratio. Actually, I’ve got a little ahead of myself in talking about these circuit details but they are really operational features so it was hard to avoid. Let’s now get down to the nitty gritty of the this sampling takes place is called the FRAME RATE and is typically 16-24ms in the 8-channel system. The eight identical input stages each contain a 3-pin plug (TB2, TB4, etc), to which the control stick pots are connected. These provide the channel allocation and servo reversing features. The second set of 3-pin plugs (TB1, TB3, etc) are used to select NORMAL or GAIN VARY modes, depending on how the asso­ciated shorting link is plugged in to short between the centre and one outside pin. Setting the shorting link on the NORMAL pair gives a fixed 1-2ms pulse width variation. Setting the Fig.1 (next page): the circuit uses an 8-input multiplexer (IC4) which is switched by counter IC5. IC4 samples all eight inputs in sequence and this creates a staircase waveform at the output of IC3a. the two comparators (IC1a & IC1b) then transform this into the pulse position modulation (PPM). March 1996  55 56  Silicon Chip March 1996  57 each individu­al channel or input in the encoder. Thus, the output of IC3a will be DC but stepped up and down (ie, a staircase), according to the settings on each of the eight inputs. This output is fed to pin 2 of comparator IC1a where it is compared with the ramp generator (IC3b) at pin 3. The output of the comparator is a series of narrow pulses whose timing, rela­tive to each other, is a function of the DC input and the ramp; so the higher the DC input, the longer the time between successive pulses. Thus, the output of the comparator is a block of eight pulses which have times between them proportional to the gain settings on the inputs. This is known as pulse position modula­ tion (PPM). Synchronising This scope photo shows how the encoder produces PPM (pulse position modulation) from the eight multiplexed input channels. The upper trace is the staircase waveform at pin 1 of IC3a (following the multiplexer) while the locked pulse waveform is from pin 7 of IC1b. The long positive pulse the sync pause. shorting link on the GAIN VARY pair provides 20-120% servo travel controllable from VR1, VR3 etc. The shorting links on TB1, TB3, etc may be each replaced with an SPDT switch which allows remote programming from the front panel. VR1, VR3, etc are gain controls and can perform multiple functions depending on how the terminal blocks TB1, TB3, etc are set up (programmed). Thus, they can provide the dual rate adjust, servo gain (ATV) and mix ratio. Mixing expansion port TB10 is the mixing expansion port and this is normally fitted with a shorting plug for the main 8-channel input leads. This is removed when the configuration module is plugged into this port. The pins for this port are double-sided and they also act as pick-up points for the mixers involved with IC6. We’ll come to those later. TB11 is the 24-channel expansion port. TB12, TB13 and TB14 are the channel number select connectors and select 8, 16 and 24 channels respectively. For example, if TB13 is shorted, then 16-channel operation is selected. The main board comes with a short­ing bar on TB12 (on the PC tracks) which must be cut if you intend to install more than eight channels. Likewise, 58  Silicon Chip R25 must be removed for more than eight channels. These connectors may be hard wired or fitted with header pins if you intend to swap backwards and forwards from 8 to 16 or 24 channels. These pins could even be wired to a 3-position switch on the front panel which would allow front panel selection of 8, 16 or 24 channels. As pointed out previously, the flexibility of this system is virtually unlimited. TB7 is the power input connector and it also carries the modulation to the RF module. Let’s start our analysis with IC5, a 4024 counter which continually feeds a series of binary numbers to the A, B & C pins of IC4. Thus, IC4 sequentially switches each of its eight inputs through to R20, the input resistor for IC3a which is a DC ampli­fier. Because IC4 is an addressable analog switch, any resistance in series with its inputs (ie, R3, R6, R9, R12, R26, etc) must be considered to be in series with R20. This total resistance will therefore determine the gain of each individual input. From this simple fact derives the magic of the Mk.22 encoder. The ratio of this total resistance (including R20) and IC3a’s feedback resis­tor R18 will determine the servo travel (GAIN) for The 8-pulse block is synchronised by the sync pause generator, IC3c, an op amp functioning as a one-shot. Each time Q4 of IC5 goes high, it charges C9 via diode D2 and R16 and the resultant high pulse from IC3a resets IC5 via R21. So IC5 starts again and switches the first of the eight inputs through to IC3a and the sequence continues. The length of the sync pause is controlled by the RC time constant of R15 & C9, which set it at 8ms. This is a very important point, particularly in 16 and 24-channel transmitters, as it gives the minimum frame (repetition) rate and thus helps to minimise servo slow down. This can arise in some servos if the servo pulse stretcher cannot cope with the long repetition rates used in the high level transmitters. The alternative system found in some transmitters is to use a fixed frame rate which must be long enough to encompass the maximum width control pulse (2ms) plus the sync pause (8ms). Thus, a 24-channel transmitter of this type would use a fixed frame rate of [(24 x 2) + 8]ms = 56ms. By contrast, the Mk.22 uses a swinging frame rate which varies between [(24 x 1) + 8]ms = 32ms to 56ms, depending upon where each of the control pots is set. In high level transmit­ters, it is a good idea to leave all unused channels set at 1ms to speed up the repetition rate. In the 8-channel transmitter, we use a frame rate which will vary from 1624ms. The form of frame rate generation used in the Mk.22 also has another advantage when changing from 8, 16 or 24 channels. As the sync pause is tacked onto the last pulse, the frame rate increase is adjusted automatically to suit the number of channels in use. For people who are concerned about achieving the minimum frame rate (maximum data refresh rate), the sync pause may be set lower but be aware that there is no standard for the sync pause in commercial receivers. Some will operate comfortably on 4-5ms but others will fly out of sync even at 6 or 7ms. From past experience, I have found 8ms to be a fairly safe time constant. As the Mk.22 Tx is intended as a replacement for all commercial transmitters, we have been a little conservative here. Unfortunately, the picture of the circuit operation present­ed so far is a lot more complex in reality. IC2a, a D-type flip­flop, is actually the controller of everything. And to further confuse things, it is not even used like a conventional D-type. Instead, it is used as an RS flipflop which is “set” by the pulse output from IC1a and “reset” by IC1b, another comparator. Neutral comparator IC1b is the neutral comparator and is set by VR2. The vol­tage from VR2 is compared with the same ramp generator signal from IC3b and this produces a similar series of narrow pulses with a 1.5ms spacing between them. So each time IC2a is set by IC1a, it is reset by IC1b a little later. IC2a not only clocks counter IC5 but it also drives the ramp generator, IC3b, which is actually an RC integrator; it “integrates” the output of IC2a and so we have a sawtooth wave­form which is locked to IC2a, to the counter and to everything else. So how do the pulses from IC3a actually get to the modula­tion output on plug TB7? The answer is that they don’t. The pulses generated by IC1b, since they are locked to everything else, actually become the modulation. Comparator IC1b drives transistor Q1 and thence D-type flipflop IC2b which functions as a monostable multi­vibrator. Its Q output will go high when ever it is clocked by comparator 2 (IC1b) and stay high for a period set by the RC network of R11 & C5 which drives the reset pin. R52 and D3 speed up the recovery time and eliminate variations in the modulation pulse length due to variations in the width of the control channels. The nominal length of the modulation pulse is set at 350µs. IC3d is the pulse shaper, an op amp integrator used to adjust the rise and fall time of the modulation pulse to the RF modulator, an important point when we come to the RF module. A correctly set modulation pulse will result in a bandwidth of around ±10kHz. IC3 is a TLC 2274, specified to provide near rail-to-rail switch­ing for the RF modulator. R55 and C4 also help to reduce the modulation rise and fall times to reduce RF harmonics. Whilst in theory C6 should provide symmetry on both leading and trailing edges, in practice we found C6 controls the leading edge slope and R55 and C4 control the trailing edge slope. YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT Mixers Op amp IC6 provides four mixers with gain set by VR8, VR10, VR12 & VR15, respectively. The two small modules at bottom right – TB17, 20 and TB21, 24 – are toggle switch control modules. TB27 and TB28 (middle left) are mixer select programming pins. These mixers are connected to the main circuit by small patch cords to the appropriate pins on the mix expand port, TB10. Another very important feature of the circuit is the voltage reference rails provided by R22, R23, R58 & R61 which are 1% resis­tors. These accurate voltage references are derived from REG1, an LP2950 low drop-out 5V regulator This regulator allows reliable operation down to 5V or less on the transmitter battery. These accurate reference rails allow servo reversing on all channels by simply reversing the control pot polarity. In the Mk.22 encoder, this is done by reversing the 3-pin socket associated with each control pot. This function could be achieved with switches but it would mean the loss of channel allocation. Channel allocation in the Mk.22 is achieved by simply connecting any pot to any channel. Channel allocation is a vitally important feature when we come to the F3B module for example, where two channels are required for ailerons and another two for flaps; using only one stick axis for each pair of channels. So there we have it, a basic no frills 8-channel encoder with expansion to SC 24 channels if required. YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 March 1996  59